The present invention relates to injection moldable PVA sponge products and a process for making such products.
Polyvinyl alcohol sponges (HPVA) usually have over 40 percent of their alcohol functions acetalized, and have a useful range of very desirable properties. They are open celled highly water absorbent porous flexible materials when moist, that wick aqueous solutions quickly. They are compressible when dry, expandable when wet, have high tensile strength, good elongation, and excellent resistance to most chemicals. They can be made bio-compatible, safe and therefore be non-toxic to the environment, and are exceptionally white in color. In some embodiments of the present invention, such sponges are more fully acetalized. These more fully acetalized sponges tend to become more rigid with reduced elongation. They are also more hydrophobic, very light in weight when dried, and thus, resemble Balsa or Cedar wood, except much stronger.
In the prior art, products derived from the less acetalized softer HVPA were mostly cut from blocks or large pieces, resulting in parts having some sharp corners and non-rounded dimensions. Also, the surface of these cut parts consisted of varying cross sections of pores, and therefore, were relatively rough on the outer surface because of the exposed pore edges.
The prior art relative to polyvinyl alcohol open cell foams (HPVA foams) describes processes that produce the material in large buns or blocks by pouring the foamed uncured resin into containers or other forms that produce large blocks. After these blocks are cured, sheets or pieces are derived therefrom by cutting and stamping processes. Both the cut surfaces and the core material have identical pore size. The cut surface, however, tends to be somewhat rougher due to the cross-sectioning of the pores, but the appearance is the same.
The surfaces of the cured large blocks of HPVA foam that are in contact with the curing container have surface skins with a different appearance than the core material. These surface skins are mostly discarded because the surfaces have a different appearance and texture from the rest of the sponge and were thought to be a nonporous skin which would inhibit water absorption.
It has been found, in accordance with the present invention, that these skins are quite able to absorb water. It has been found that they consist of a porous membranous skin. These skins conform to the interface of the curing container and the foamed resin. This skin appears nonporous because of its smooth surface. The surface also appears microscaled because it consists of much smaller pores than the core material. On further investigation, however, it was determined that the skin was surprisingly still porous. These smaller open pored smooth surfaced sponges may therefore fulfill the need for improved wound release non-abrasiveness or receiving printing or decorating without compromising the absorption functionality of the sponge body, and these surprising characteristics are utilized by the present invention.
HPVA sponges produced by the prior art for medical packing and protective applications are cut from blocks, and therefore, cannot faithfully fit the rounded contours of a body cavity since the body has no sharp corners. These sponges also have relatively rough textured surfaces into which tissue ingrowth can occur and be abrasive to delicate tissue, e.g. the surface of the brain. Adherence of tissue causes difficulty in removal, this being especially important in surgical packings. To overcome the tissue adherence problem in prior art HPVA sponges, ointments such as petroleum jelly, are coated onto the packing surface, but these ointments reduce absorption and can produce foreign body reactions, e.g. myospherulosis infections. Smaller pore cut HPVA sponges may produce less adherence, but their increased density necessary to produce the smaller pores reduces softness, and liquid holding capacity. The present invention overcomes these problems without necessitating ointments.
Another use for the novel molded HPVA products of the present invention is as a toy. Prior art toys made of other sponge materials, e.g. absorbent cellulose or open cell urethane sponges, do not have much physical or color detail resemblance to the natural model they represent and suffers because they are relatively crude in design, or possess a rough or somewhat sealed surface. Therefore, they cannot receive good decorating detail or absorb enough water. The HPVA sponge of this invention, however, has a surface that permits faithfully detailed decoration and physical appearance detail, e.g., replicating water fowl or other animals. The result is a uniquely attractive and safe bath or play toy. The products of this invention are also odorfree, non-toxic, strong and functional as a true bath sponge.
A further use for the molded PVA sponge of the invention is as a fishing lure. HPVA polymer low density impregnated sponge lures have a unique advantage over the prior art soft lures by having more life-like action in the water due to their high flexibility, and high water content. These lures also have relatively high strength to give improved tear resistance on the hook, and a close to neutral buoyancy so that it neither sinks nor floats on the water, but remains substantially suspended within the body of water. The prior art soft lures which all tend to sink are mostly made of highly plasticized polyvinyl chloride which is heavier than water. They are also relatively weak which can cause propagation of a tear around the hook, and loss of the lure.
The smooth hydrophilic surface of the HPVA sponge fishing lure of the invention more resembles natural bait than the solid greasy plasticized polyvinyl chloride soft lures of the prior art because it has an appearance and feel of a moist smooth slippery surfaced live bait skin membrane. This is because of the HPVA sponge fishing lures 80-90% open or connected pore volume which when filled with water causes it to float and move naturally, e.g. live baits are also mostly water. The high internal pore volume of the sponge lure also enables the incorporation of large amounts of fish attractants within the voids of the cells of the porous body which will better resist washing out and thus last longer in the water when compared to the small amounts of topically applied fish attractants now used with the prior art lures. The attractants do not wash out easily from the molded HPVA sponge lure because they are entrained homogeneously throughout all the interstitial small pore spaces of the sponge body.
Other benefits of a molded HPVA lure as compared to present vinyl lures are improved strength and longevity with longer hold time by the fish due to the softness and taste of the impregnated sponge thus allowing the fisherman more time to set the hook. HPVA molded sponge lures are environmentally safe, and can be dyed and decorated easily with brilliant colors. They have indefinite shelf life, are odor free until impregnated with fish attractant, and cannot be dislodged easily by the fish as the fine teeth of the fish tend to get entangled in the soft porous surface of the lure.
The prior art processes for making PVA sponge differs from the present invention. Most prior art processes warm the premix to make PVA foam sponges. One reason for preheating is to partially polymerize the resin mix to obtain a viscous more stable foam which stabilizes the pore size.
Another objective for warming the premix is to form blocks of HPVA with minimum shrinkage or collapse of the cured foam and thereby obtain reasonable curing times of 12-24 hours. Prior art processes disclose mixing and pouring resins at 110.degree.-125.degree. F. followed by about 16 hours cure time. If greater heat is utilized in the premix stage to obtain faster cures, the resin would cure in the mixing pot before it could be poured. In much of the prior art, the 110.degree.-125.degree. F. mixing range was found to be a practical range to effect cures within about 16 hours commensurate with acceptable shrinkage of the cured block. The temperature and state of the premix was important for some products whose pore size was critical. Therefore the in situ trapped heat is necessary to also final cure the foamed resin homogeneously throughout because when pouring into a container an air entraining resin mix becomes an insulating body, thus preventing additional externally applied heat from easily penetrating quickly and uniformly throughout the core of the block. If high temperatures were subsequently used to drive the heat more quickly in from the outside of the mold into the foamed resin interior to raise the temperature and thereby effect faster cures, e.g. containers placed in a hot oven, the surface of the resin in contact with the mold would soon deteriorate before the heat could travel by conduction into the block. Deterioration of curing HPVA polymer occurs at high temperatures because the sulfuric acid at high temperature, if allowed to concentrate by evaporation of its water content, will tend to cause dehydration and carbonization of the resin resulting in undesirable darkening and physical property deterioration of the polymer.
A second reason for warming the premix in the prior art is to prevent the uncured air entrapping foam from collapsing once it is poured in the curing container due to the entrapped air prematurely escaping. The foam that is produced in this warm premixing environment soon begins to cure producing a more viscous or partially cured and therefore strengthened precured foam mix. Due to this initial curing, the foam is given greater internal strength and stability since the air now has less tendency to escape from the more viscous mix. Therefore there is less tendency for foam collapse in the curing container and the size of the block is maintained close to its original container volume producing more uniform pores. However, this warm mix process also sacrifices the working life of the resin mix. The warm foam is introduced into or poured almost immediately into its container because it is now curing and thickening rapidly. Some shrinkage away from the container does occur but mainly because the chemical reaction produces water as a by-product and the new water insoluble HPVA foam takes up less volume than the uncured premix.
The prior art utilization of heat while mixing to increase premix viscosity, to prevent foam collapse and loss of pore size uniformity, and pouring the resin when warm into the container to reduce shrinkage, causes decreasing pot life. This prior art procedure therefore cannot be used to make a long pot-lived mass production moldable type of HPVA resin formulation. The latter process of this invention also requires premixes with lower viscosity and long pot life thus having the ability to fill small cavities with detailed interstitial spaces which can duplicate, with fidelity, the surfaces of the mold. Whereas, the prior art teaches pouring a warm, thick, partially cured resin mix into room temperature containers, this invention teaches pouring a very flowable, relatively uncured cold resin mix into molds that are hot or will be soon heated.
The molded surface skin's properties of this invention differ from other resinous molded foam surfaces because other moldable plastic foams result in having skinned surfaces which are of a nonporous or impermeable nature, e.g. Polyvinyl chloride (PVC foam used in car seats and upholstery coverings). The surface of HPVA sponges was also found to reproduce with fidelity small imperfections of the container wall in which it was cured such as scratches or gouges. From these observations it was concluded that HPVA foam material is capable of being produced in molded parts with surface detail having porous relatively smooth surfaces that can still absorb and pass water into the interior of the sponge part and be capable of being decorated with colors. However, these desirable properties could not be achieved economically or mass produced by conventional HPVA technology. Thus, the prior art HPVA resin formulations and processes must be changed to achieve low viscosity and long working life foams and further to be adaptable to new molding processing techniques.
This invention seeks to overcome the disadvantages of the prior art to achieve smooth contoured surfaces by employing a process and formulas that mass produce HPVA sponge parts that have rounded dimensions and smooth porous surfaces while still retaining all of the desirable properties of the HPVA sponge polymer.